WASHINGTON (CNN) --Responding to the build-up of U.S. forces in the Persian Gulf, the Department of Defense has activated the Civil Reserve Air Fleet, which allows the U.S. military to transport troops and equipment by commercial aircraft, a spokesman for the U.S. Transportation Command said Saturday.

Secretary of Defense Donald Rumsfeld authorized the Stage One activation Saturday, according to a news release from the Defense Department. Stage One authorization is the lowest activation level, and occurs with "minor regional crises."

Though the activation means 22 U.S. airline companies and their 78 commercial aircraft may now be called upon to move large numbers of troops and cargo for the U.S. military, Navy Capt. Steve Honda said the commander of the Transportation Command, Air Force Gen. John Handy, is only activating 47 passenger aircraft.

Handy may still choose to activate the other 31 aircraft, which are all cargo planes, Honda said.

"This measure is necessary due to increased operations associated with the build-up of U.S. forces in the Persian Gulf region," the news release said.

The Civil Reserve Air Fleet includes aircraft from dozens of companies, ranging from Delta Air Lines and Federal Express to Reno Air and Sun Country Airlines.

The airlines receive large peacetime contracts as incentives to participate in the CRAF program. The carriers operate and maintain their planes at their own expense, with the military controlling their missions.

When notified of a call-up, each carrier has 24-48 hours to have its aircraft ready for a mission.